Air-source heat pumps are a common heating source in the southern United States and in many places around the globe. Heat pumps collect and move heat into an enclosed space in the heating mode, or expel heat from the enclosed space in the cooling mode. Heat pump systems use a closed refrigeration circuit for circulating a working fluid or refrigerant to move thermal energy through the circuit by collecting it in one part of the circuit and moving it to another.
For example, a refrigeration circuit can use a compressor to raise the temperature and pressure of the refrigerant before delivering it to a condensing unit. Heat is dissipated from the condensing unit as the refrigerant condenses and changes phase from a hot high-pressure vapor to a warm high-pressure liquid. The high pressure warm refrigerant may then pass through a metering device (also called an “expansion valve”) which can reduce the pressure of the working fluid before it enters an evaporating unit. Because of this pressure reduction, the working fluid changes phase from a warm high-pressure liquid to a two-phase mixture of liquid and vapor at a lower temperature and pressure. During this phase change, some of the warm liquid condensate quickly boils away (or “flashes”) to a gas thereby absorbing enough heat from the working fluid to cool the remaining liquid. The remaining liquid then evaporates by absorbing heat from an external medium outside the evaporator such as air, the ground, a supply of fluid such as water, or some other heat source. The evaporated refrigerant reenters the compressor, and the cycle is repeated during normal operations.
In most residential settings, a heat pump system can either heat or cool an enclosed space by selectively controlling the flow of refrigerant using one or more valves and by using reversible metering devices in the circuit. These metering devices are configured to cause a substantial pressure reduction if working fluid flows one way while allowing the fluid to pass without a substantial pressure reduction if the fluid flows in the opposite direction. Typically this substantial pressure reduction occurs when the working fluid passes downstream from a metering device into a nearby heat exchanger (positioned either inside or outside the enclosed space). Thus in most such systems the heat exchanger immediately downstream from the metering device is operating as an evaporator collecting heat energy from an external medium to evaporate the refrigerant. In the cooling mode, the heat exchanger operating as an evaporator is positioned indoors to collect heat from within the enclosed space so that it may be moved along the circuit and expelled outside the enclosed space through another heat exchanger operating as a condenser. On the contrary, in the heating mode, the heat exchanger operating as an evaporator is positioned outdoors to collect heat from outside the enclosed space so that the heat may be moved through the circuit and expelled indoors through the other indoor heat exchanger now operating as a condenser. Thus such systems are “reversible” in that the indoor and outdoor heat exchangers can alternately operate either as an evaporator or a condenser depending on whether the system is operating in a heating mode or cooling mode.
In such heat pump systems, multiple metering devices can regulate the flow of the working fluid using a sensing device to detect the temperature of the working fluid vapors leaving the evaporator. The metering device can respond by opening when vapors leaving the evaporator are too hot, thus allow more refrigerant into the evaporator lowering its temperature, or by closing when the vapors are too cold to keep the quantity of refrigerant lower and temperatures higher. In this way, metering devices can control the temperature of the evaporator by regulating the flow of refrigerant into the evaporator depending on the load on the system and the rate of evaporation. Metering devices can then be calibrated according to the working fluid in use and the application of the refrigeration circuit (heating or cooling) to ensure working fluid in the liquid phase does not enter the compressor which can damage it.
As described above, some amount of working fluid immediately boils away when the metering device reduces the pressure because the working fluid cannot remain a liquid at a temperature higher than the boiling temperature corresponding to the lower pressure in the evaporator. The warm condensed liquid can no longer remain a liquid at the reduced pressures causing some part of the condensed liquid to evaporate and cool the remaining fluid in the liquid phase.
Situations can arise where this phase change may occur before the working fluid enters the metering device. This can occur, for example if the warm condensed liquid decreases in pressure or increases in temperature as it passes through the lines leading from the condenser to the metering device upstream from and adjacent to the evaporator. Even though these changes may be minor, they may be sufficient to cause vapor phase working fluid bubbles to form within the lines leading to the metering device thus causing gas to enter and pass through the metering device.
Such situations are usually disadvantageous to the smooth functioning of the refrigeration circuit. When a two-phase mixture of liquid and gas working fluid enters the metering device, the hotter gases generally pass quickly through the evaporator and into the compressor. The temperature sensor at the evaporator outlet may sense the higher temperature of the passing vapor and cause the metering device to react quickly as if a large heat load were suddenly present thus allowing a surge of condensed liquid into the evaporator. However, just as quickly, the bubble of hot vapor moves past the sensor, and the cooler evaporated vapor moves by the sensor causing the metering device to quickly close again. If the cause of the vapor phase bubbles in warm condensate is not remedied, high temperature vapor pockets may continue to pass through the evaporator at irregular intervals causing a frequent and erratic opening and closing of the metering device. Such a condition is sometimes referred to as “a hunting expansion valve” condition causing continuous overfeeding and starving of the refrigerant flow to the evaporator. This can result in erratic performance, abnormal wear on the metering device, and inefficiencies in overall performance of the system.